1. Field of the Invention
The present invention relates generally to lithographic projection apparatus and more specifically to lithographic projection apparatus incorporating aberration correction. 2. Background of the Related Art
The term xe2x80x9cpatterning structurexe2x80x9d as here employed should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning structure include:
A Mask.
The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A Programmable Mirror Array.
An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-adressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
A Programmable LCD Array.
An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning structure as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer.
If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. An important aspect of the performance of a lithographic projection apparatus is the so-called alignment accuracy. In the manufacture of an IC, which is often built up of several (tens of) patterned layers, it is extremely important to ensure that the overlay juxtaposition) of the various stacked layers is as accurate as possible. For this purpose, a small reference mark is provided at one or more positions on the wafer, thus defining the origin of a co-ordinate system on the wafer; using optical and electronic means (referred to hereinafter as xe2x80x9calignment systemxe2x80x9d), this mark can then be re-located each time a new layer has to be juxtaposed on an existing layer, and can be used as an alignment reference.
Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
The phenomenon of lens heating can occur in the projection system of a lithographic projection apparatus. The projection lens becomes slightly heated by the projection beam radiation during exposures. As a result of this heating, refractive index changes occur, and a certain expansion of lens elements occurs, causing subtle changes in the geometric form of those elements, with an attendant change in their optical properties. This can result in the occurrence of new lens aberrations, or a change in existing aberrations. Because the occurrence or worsening of these aberrations depends on such matters as the particular lens geometry, lens material, projection wavelength, light source power, target portion size, and so on, the phenomenon is intrinsically unpredictable.
Lens heating has always occurred to some extent in lithographic projection apparatus. However, with the trend to integrating an ever-increasing number of electronic components in an IC, and to increase the manufacturing throughput, shorter wavelength radiation, such as deep ultraviolet (DUV), 193 nm and 157 nm radiation has been used, as well as high-power radiation sources, such as 3-6 kW Mercury-arc lamps and excimer lasers with a power of 10 to 20 W, which together with the reduction in feature size have made lens heating a more serious problem. The problem is generally worse in scanners than in steppers because, in a stepper, substantially the whole (circular) cross-section of each lens element is irradiated, whereas, in a scanner, generally only a slit-shaped portion of the lens elements is irradiated; consequently, the effect in a scanner is far more differential than in a stepper, thereby resulting in the occurrence of new lens aberrations.
It is a further problem that the above problems cannot be satisfactorily prevented by measures such as the use of lens cooling jackets.
It is an object of the present invention to provide an improved imaging method and apparatus in which said problems are alleviated.
Accordingly, the present invention provides a method of operating a lithographic projection apparatus as specified in the opening paragraph, the method including calculating a change in aberration effect in said projection system, due to heating effects, as a function of time, based on at least one set of predetermined parameters, and adjusting the lithographic projection apparatus to compensate for the calculated change in aberration effect.
Preferably said parameters are obtained by a calibration step. The calibration step may comprise a coarse calibration followed by at least one fine calibration, where the coarse calibration yields a first estimate of at least a subset of the parameters. Said first estimate can be used as an input for the subsequent fine calibration. However, first estimates of parameters may, for example, be available from design data or empirical evidence. In this latter case the calibration may comprise a single (fine) calibration step.
Preferably a (fine) calibration is successively performed at a plurality of different illumination settings and/or with a plurality of different patterns as provided by patterning structure and/or with a plurality of different substrates provided on said substrate table, wherein a set of parameters is obtained for each calibration, and the sets of parameters from the calibrations are stored in a database.
Said plurality of different illumination settings may comprise different numerical aperture settings and/or sigma settings (defined below), illumination modes or telecentricity modes; furthermore, one may use various types and sizes of test structures on one or more test masks to create different diffraction effects in the projection system. All such variation should be interpreted as falling within the meaning of the phrase xe2x80x9cdifferent illumination settingsxe2x80x9d of the radiation system, as used in this text. The term xe2x80x9csigma ("sgr") settingxe2x80x9d refers to the outer and/or inner radial extent (commonly referred to as "sgr"-outer and "sgr"-inner, respectively) of the intensity distribution of the projection beam in a pupil plane of the radiation system, normalized with respect to the maximum radius of the pupil. Thus, a sigma value of 1 represents illumination with a radius at the pupil equal to the maximum radius of the pupil. The term xe2x80x9cillumination modexe2x80x9d denotes the spatial distribution of the radiation at the pupil, which may, for example, be disc-shaped, annular (which would be characterized by inner and outer sigma settings), quadrupolar, dipolar, or soft-multipolar (including some radiation flux in between and/or around the poles), for example. More information on illumination modes can be gleaned, for example, from European Patent Application No. 00311007.9, incorporated herein by reference. The term xe2x80x9ctelecentricity modesxe2x80x9d encompasses configuring the radiation system and/or projection system telecentrically and/or with varying degrees of non-telecentricity, for example by the use of prisms on top of a reticle to tilt the illumination profile. These different illumination settings can be selected conveniently in a lithographic projection apparatus.
Preferably some of said parameters are common between sets, which enables fewer calibration steps to be performed and reduces the database size.
Advantageously, for operating said lithographic projection apparatus under conditions for which a set of parameters has not been predetermined, said calculating step comprises interpolating or extrapolating at least one of said parameters based on parameters obtained for the projection apparatus under different conditions.
Preferably, said aberration effect comprises at least one of focus drift, field curvature, magnification drift, third-order distortion, spherical aberration, comatic aberration, on-axis astigmatism, asymmetric magnification and combinations thereof.
Preferably, said adjusting step comprises adjusting at least one of: the position of the support structure along the optical axis of the projection system, the rotational orientation of the support structure, the position of the substrate table along said optical axis, the rotational orientation of the substrate table, the position along said optical axis of one or more moveable lens elements comprised in said projection system, the degree of decentering with respect to said optical axis of one or more moveable lens elements comprised in said projection system, the central wavelength of the projection beam, and saddle-like deformation of one or more lens elements comprised in said projection system using edge actuators.
The present invention also provides a lithographic projection apparatus for imaging a pattern as provided by patterning structure onto a substrate provided with a radiation-sensitive layer, the apparatus including a radiation system for providing a projection beam of radiation, a support structure for supporting patterning structure, the patterning structure serving to pattern the projection beam according to a desired pattern, a substrate table for holding a substrate, a projection system for projecting the patterned beam onto a target portion of the substrate, and illumination setting means for providing a plurality of different illumination settings of said radiation system, a memory to store at least one set of predetermined parameters, a processor to calculate a change in aberration effect in said projection system, due to heating effects, as a function of time, based on a set of said parameters stored in said memory means and a controller to adjust at least one component of said apparatus to compensate for the calculated change in aberration effect.
According to a further aspect of the invention there is provided a device manufacturing method including providing a substrate that is at least partially covered by a layer of radiation-sensitive material, providing a projection beam of radiation using a radiation system, using patterning structure to endow the projection beam with a pattern in its cross-section, projecting the patterned beam of radiation onto a target portion of the layer of radiation-sensitive material, calculating, during series of repeated exposures of corresponding series of target portions, a change in aberration effect in said projection system as a function of time, based on at least one set of predetermined parameters, and adjusting the lithographic projection apparatus to compensate for the calculated change in aberration effect.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range 5-20 nm). The term xe2x80x9cheatingxe2x80x9d as used throughout this text, should, in principle, be interpreted as encompassing cooling also.